With a shape like that, Craig's suggestion is right on for most bang for your buck!
Also: undertray? I'm betting the motor is in the back, so a full tray may be feasible.
Maybe you've seen Phil's VW Van mod thread:
http://ecomodder.com/forum/showthrea...-van-8675.html
I'd also suggest instrumentation. Get yourself an MPGuino. It will work, provided the engine is fuel injected.